Gas canisters were ignited near to a mental hospital.
The canisters exploded causing significant damage to the former Harperbury Hospital site in Radlett.
Police are appealing for information after the incident at 11.50pm on Friday.
They said intruders entered Kingsley Green Harper House, in Harper Lane, during which time a fire occurred which ignited some gas canisters on the site.
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DC Mark Chipchase of the St Albans Local Crime Unit who is investigating, said: “This fire set off some highly combustible canisters and could have led to serious injuries or even fatalities.
"It is illegal to trespass within the derelict buildings at the Harperbury Hospital site, and the site is a serious health and safety risk as it is in varying states of disrepair.
"We are working with the hospital to identify the intruders and appealing for anyone who witnessed this incident or saw anyone accessing the site to get in touch.”
Anyone with information is asked to contact Hertfordshire Constabulary via the non-emergency number 101, quoting crime reference F3/17/1047 or online via www.contacthertspolice.uk.
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